Amplitude control



Feb. 22, 1949. D, s, RENNER AMPLITUDE CONTROL Filed NOV. 5, l1942 MWI/V6.' REM/il? INVENTOR. @im @y 17M Patented Feb. 22, 1949 2,462,551 AMPLITUDE CONTROL Darwin S. Renner, Dallas, Tex., assignor to Geophysical Service Incorporated, a corporation of Delaware Application November 5, 1942, Serial No. 464,607

1 Claim.

This invention relates to automatic gain or amplication control, and more particularly to an amplifier system including an electrically variable attenuator and control means therefor so that for a variable input a substantially constant output may be had with high iidelity.

proposed to control amplitude automatically and within limits by altering tube bias so that gain control is effected by shifting the working point on the characteristic curve of the thermionic tube. Such mechanism for gain control produces amplitude distortion or, in other words, distortion in reproduction of amplitudes in the diierent phases of a vibratory event. This distortion is severe when high attenuation is attempted and tube operation occurs proximate the knee of a characteristic curve of the tube.

both phase and frequency distortion in view of the fact that the alternating current resistance of tubes varies because of bias variations. It is these reasons, and others, the tube bias method of amplitude or gain control is unsatisfactory and in particular is this true in the ield of seismic geophysics where large variations in amplitude are to be controlled.

In this particular eld of utility of the present invention a frequency spectrum between 2.5 cycles per second and approximately 200 cycles per second is involved, and the frequency band of particular interest lies approximately between 18 cycles per second and '7G cycles per second. Additionally, a visual record is produced as distinguished from the reproduction of a vibratory event in the form of soundy and for best results there is required accurate and faithful reproduction of the vibratory event and also proper gain control over an extremely wide range of amplitudes. While the utility of the invention in the eld of seismic geophysics is specifically referred to in the present disclosure, it is to be understood that the invention is not coniined to this paras the invention is of utility in gain control and faithful reproduction of vibratory events in any of the various fields.

The primary object of the invention is to overcome the difliculties to which reference has above been made.

Another object is to provide faithful reproducof a vibratory event and accurate control of the amplitude of the amplified vibrations of the event.

Still another object is to provide a device of the class described that will operate satisfactorily at any particular frequency or over a large band of frequencies.

Another object is to provide an amplitude control device having a large control range.

A further object is to provide an attenuator control network of which the attenuation is unaiected by the signal transmitted throughout the network.

A still further object tude control device which 1s rugged and which throughout the long life thereo Another object is to provide a device of the class described which is free from amplitude distortion.

A still further which eliminates the introduction oi spurious signals and which also avoids the introduction of phase and frequency distortion in the ampliiied signals.

A more specific object of the invention is to Provide an automatic Still another object is to provide a device for gain or amplitude control of particular utility 'n the field of seismic geophysics and operable to secure usable records of all phases of a seismic event.

Another object is to provide an automatic gain control system including a substantially constant gain amplier and a cooperating variable attenuator network controlled thereby to, in turn, control the amplitude of the output of the amplifier.

The foregoing are primary objects and will,

objects, be more fully apparent from the following description taken in connection with the accompanying drawings in which:

Fig. l is a schematic diagram showing the essential elements of an automatic gain control device or system constructed in accordance with the invention;

Fig. 2 is a circuit diagram showing in greater detail the various elements of Fig. 1.

Referring to Fig. l of the drawings which diahereinafter, the attenuator netthrough the conductors 4 to the amplifier 5 is controlled in a manner poses of the invention. The output of the ampliiier 5 is transmitted through conductors 6 to the output circuits 1 and thence to the output terminals 2 which, as above indicated, are connected to any suitable device such as a speaker, a transmission line or in the case of seismic prospecting, to the galvanometer or other recording equipment.

The system generally described above may be considered as a composite transmission line including an attenuator control circuit Il) connected therein at a suitable point between the input terminals I and the output terminals 2. Preferably this control circuit is connected to the amplifier 5 in a manner that the controlling current supplied through the conductors 9 to the network 3 provides such attenuation of the vibrations that the input to the amplifier 5 is such that the vibrations at the output terminals 2 are of a desired amplitude.

As already pointed out, the attenuator network 3 comprises a, plurality of resistors which are shown more fully in Fig. 2 as comprising line resistors I5 and I6 interconnected at point I1 and resistors I8 and I9 in parallel therewith and interconnected at the point 2li which is also grounded inasmuch as a grounded return is utilized in the system. Input to the terminals I energizes the primary 2l of the transformer 22 of which the secondary 23 is connectedto the respective above mentioned series resistors at the points 24 and 25.

rEhe attenuator may comprise any suitable number of sections, and two such sections are shown for convenience. In addition tothe resistors I5 and I8, the first section of the network also includes bridging resistors 26 and 21 of which the former is connected at points 210 and 24 and the latter is connected at points I'I and 25. In a similar manner vthe second section comprising resistors I5 and I9 includes bridging resistors 2B and 29 of which the former is connected to points I-'I and 30 and the latter is connected between points 20 and 3|.

The output of the attenuator from Apoints 3B and 3l is connected to the primary terminals of the output transformer 32, and the output from the secondary terminals of the transformer is transmitted through the conductors 4 to the amplifier 5 as already explained.

To change the attenuation of the network 3, the value of one or more of the resistors thereof must be changed and such change must be made in a manner that the transmitted signal will not be undesirably affected. A convenient method of eiecting the desired resistance change is that of controlling the temperature of certain of the resistors and hence taking advantage of the temperature coeiiicient of resistance of the resistors. Such temperature change may be effected by thermal conduction or by direct electrical conto accomplish the pur' -side but under most circumstances:

That is, the chosen frequency fh is sufliciently removed from the signal band that its separation from the frequencies f1 and f2, both from the potential and circuit standpoints, is accomplished .with little or no extra precaution.

Preferably, though not necessarily, all the resistors of the network 3- are in the form of tungsten lamps and the elements 25, 21, 28 and 29 are equally heated in a manner hereinafter described. When no heating power is supplied to the heated elements or resistors the network has a maximum attenuation. It seems apparent that the sections of an illustrated two-section network pass monotonically to lower attenuations as power is applied to the elements.

Regardless of the exact nature of the resistor elements 26 to 29 inclusive, the power necessary to effect heating thereof should be as small as possible. To this end these resistors are preferably filamentary and enclosed, as in an evacuated envelope, to avoid atmospheric convection. Inexpensive and readily available panel bulbs requiring only one-tenth of a watt, with straight line, low temperature tungsten filaments may be used and are practically indestructible. A resistance change of as much as 10 to 1 can be realized from such bulbs without energizing the bulbs to much more than one-half their rated power. It is to lbe understood that all of the resistors of the network 3 may take the suggested or similar form and heating may be supplied as hereinafter described to the selected resistors.

The resistors chosen will normally satisfy the requirement that the power to heat them at all will greatly exceed the maximum signal power present within them.

The manner of effecting desired heating of the selected resistors and for amplification of the signal output from the attenuator net 3 will now be described. II'he output from the terminals of the secondary of the transformer 32 constitutes the input to the tube 40 of the amplier 5 which includes tube 4I and output tube 43 of which the plate current is supplied through conductor 6 to the primary 44 of transformer 45 in the output circuit 1. Energy induced in the secondary 46 of the transformer e5 is supplied to the terminals 2 and thence to the recorder, sound reproducer or other mechanism, depending upon the desired use of the amplified vibrations.

It is to be understood that while certain elements are shown as encompassed within the ampliiier 5 or within the control unit I0, yet the invention is not confined to this unitized grouping since obviously the various elements may be grouped in any desired number of units and groupings without departing from the invention. For example, thermionic tube 42 shown as a part of the unit Ill cooperates with resistors 41, 48 and capacitance 49, shown as part of unit 5,-to serve as an amplifier for the control of potential taken from the terminal I. In a similar manner the vacuum tube 55 and its associated circuit serves as an amplifier for the potential taken from the point 58 in amplifier unit 5.

The circuit indicated generally as serves as a mixer for the amplied voltages which are supplied to diode 53 for conversion of the mixed potential to a direct potential. The operation of this mixer circuit comprehends the introduction or" signals from the upper part of the tube 55 and hence from the point in the amplier unit 5 and from the tube 42. This signal is introduced to the common impedance comprising resistor E8, condenser B9 and transformer 19. Thus, the signals from the two sources are mixed in the common impedance and this mixed signal is presented to the diode 53 for rectication.

The magnitude of the rectied potential is directly proportional to the magnitude of the sum of the mixed signals, and such rectied potential exists across the resistors 67 and S5, of which the latter has the greater value. The condenser H acts as a lter condenser for the direct potential and also introduces a time constant into the circuit.

Due to the connection to the point 57 and thence to the grids of the amplifier tube 56, there is applied to these grids the rectified and filtered potential plus the relatively small xed potential applied to the terminal C-. This rectified and ltered potential, which is proportional to the amplitude of the mixed signal, is of a negative polarity to the grids. Thus, the grid potential of the amplier tube 55 is proportional to the amplitude of the mixed signal.

Tube 55 and the associated circuit comprises an oscillator with a constant output and frequency, and the output thereof is supplied to the the mixing circuit above described. Inasmuch as the control potential is of a negative polarity to the grid of the tube 56, the output from such tube is inversely proportional to the amplitude of the The output from the amplifier 56 is applied through the conductor cable 8 to the resistors of the attenuator network 3, such cable comprising conductors 24', 25', il', 30', 3i', and 20' which are connected to the respective, like, unprimed points of the resistors of the attenuator Since the attenuation of the attenuator network rises as the control power is decreased, attenuation increases as the amplitude of the signal increases, as required.

The mechanisms thus far described desirably control the output at the terminals 2 during all In the operation of the attenuation control circuit 60,

nection with the rectified mixed signal, except that adjustment of the controls 6| and 62 produces the eiiect in the absence of any signal. Thus, an attenuation can be set up in the attenuator network 3 by adjusting these controls. This manually set attenuation will be effective on the rst impulses of signal.

After a signal is present it is impressed upon the dio-de 'i2 through the circuit 'Il and the re sultant direct potential on the resistor .'3 biases the grid of the tube 64 beyond the cut-off and thus shuts oi the plate current from the tube S4 through resistors 6l and 65. At the same time, the diode 53 takes over as already described and effects the desired control throughout the signal intervals.

After the signals have ceased to exist, there will obviously be no further rectiiication at the diode 72 and the charge on the condensers '14 will leak off and the tube 84 will once again pass the plate current as set by the controls 8l and 62, and the system wil] have returned to a condition of readiness for the next signal train.

It seems apparent that when the signal input at terminals i is substantially constant, the gain within the system is substantially constant and adjustments are such that the signal output at terminals 2 have a desired amplitude. When, however, a signal of high initial amplitude is to be received, as in seismic prospecting, the controls 6l and 02 are manually operated to of ampliiier 58, whereby the temperature of the respective linear passive resistors in the attenuator unit 3 is desirably controlled.

Broadly the invention comprehends an automatic gain or amplification control which comprehends the utilization of an automatically variable attenuator and control means therefor to maintain a substantially constant, high delity output.

What is claimed is:

In an automatic gain control system for a vibratory event in a seismic amplifier the combination adapted to adjust attenuation by heating thereof, and means including a source of alternating current of a frequency widely displaced from the frequencies of the vibratory event, said source being connected to said resistors for conductively varying the heating of the resistors in a predetermined manner to control the output of the network.

DARWIN S. RENNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,035,263 Cushman et al. Mar. 24, 1936 2,084,135 Ford June 15, 1937 2,087,950 Holden July 27, 1937 2,137,020 Luck Nov. 15, 1938 2,178,333 Blair Oct. 31, 1939 

